As recombinant DNA biotechnology has developed in recent years, the controlled production by cells of an enormous variety of useful polypeptides has become possible. Many eukaryotic polypeptides, for example human growth hormone, leukocyte interferons, human insulin and human proinsulin have been produced by various microorganisms. The continued application of techniques already in hand is expected in the future to permit recombinant production of a variety of other useful polypeptide products.
The basic techniques employed in the field of recombinant technology are known by those of skill in the art. The elements desirably present for the practice of recombinant DNA biotechnology include, but are not limited to:
(1) a gene encoding one or more desired polypeptide(s), operably linked (operably linked refers to a juxtaposition wherein the components are configured so as to perform their usual function) with adequate control sequences required for expression in the host cell;
(2) a vector, usually a plasmid into which a nucleotide sequence can be inserted; a vector is any nucleotide sequence-containing construct capable of transforming a host;
(3) a suitable host into which the desired nucleotide sequence can be transferred, where the host also has the cellular apparatus to allow expression of the information coded for by the transferred nucleotide sequence.
A basic element employed in recombinant technology is the plasmid, which is circular extrachromosomal double-stranded DNA first found in microorganisms. Plasmids have been found to occur in multiple copies per cell. In addition to naturally occurring plasmids, a variety of man-made plasmids have been prepared. Included in the plasmid is information required for plasmid reproduction, i.e., an autonomous replicating sequence and/or an origin of replication. One or more means of phenotypically selecting the plasmid in transformed cells may also be included in the information encoded in the plasmid. The phenotypic or marker selection characteristics, such as resistance to antibiotics, permit clones of the host cell containing the plasmid of interest to be recognized and selected for by preferential growth of the cells in selective media. Vectors or plasmids may be specifically cleaved by one or more restriction endonucleases or restriction enzymes, each of which recognizes a specific nucleotide sequence. Thereafter, a regulatory region operably linked to a heterologous gene, i.e., a gene not naturally occurring in combination with the regulatory region, or other nucleotide sequences may be inserted by operably linking the desired genetic material at the cleavage site or at reconstructed ends adjacent to the cleavage site.
The vector is then introduced into a host cell, where its nucleotide sequence may direct the host to perform various processes or functions. A few examples include expressing heterologous polypeptides or over-expressing homologous or heterologous polypeptides. The process of nucleotide introduction into the host cell is generally termed transformation. Large quantities of the vector may be obtained by introducing the vector into a suitable host to increase its copy number. A host cell commonly used to increase the copy number of the vector is E. coli. The vectors are then isolated from the first host and introduced into a second host cell in which the desired vector-directed activities will occur, for example the production of a polypeptide. The production of an end product from DNA in this fashion is referred to as expression. When the gene is properly inserted in the vector with reference to the portions of the vector which govern transcription and translation of the encoded nucleotide sequence, the resulting vector can be used to direct the production of the polypeptide sequence for which the inserted gene codes.
Expression is controlled by a regulatory region. Regulatory regions are heterogeneous nucleotide sequences which respond to various stimuli and affect the frequency of RNA transcription. Expression may be switched on or off in response to stimuli. Expression being switched on in response to a stimuli is commonly referred to as derepression or induction. A few examples of inducible expression systems are the AOX1 system in Pichia pastoris, the estrogen systems in Xenopus laevis, and the metallothionein systems in monkeys, humans, hamsters, mice and rats. Inducible expression systems are usually under more stringent control than constitutive systems. These systems are well suited for genetic engineering purposes.
In practice, the use of recombinant DNA biotechnology may create cells capable of expressing heterologous nucleotide sequences. Heterologous nucleotide sequences are nucleotide sequences which do not naturally occur in the host. Examples of which would be new combinations of regulatory regions naturally occurring within the host with structural genes not naturally associated with this regulatory region. Another example would be the combination of a regulatory region with a gene not naturally occurring in the host. The heterologous polypeptide may be produced as a fusion polypeptide, i.e., a heterologous polypeptide fused to a portion of the amino acid sequence of a homologous or heterologous polypeptide. The initially obtained fusion polypeptide product is sometimes produced in an inactive form until the fused polypeptide is cleaved in an extracellular environment.
As was previously disclosed in European patent application No. 86114700.7 (incorporated herein by reference) the Pichia pastoris genome encodes two functional alcohol oxidase genes AOX1 and AOX2.
We have now discovered a 5' regulatory region associated with the AOX2 structural gene. This regulatory region is inducible by methanol or by carbon source starvation. However, the maximum level of expression from the AOX2 regulatory region is only about 5%-11% of that of the AOX1 regulatory region (AOX1 gene was disclosed in European patent application No. 85201235.0 incorporated herein by reference).
The AOX2 regulatory region can be employed to express heterologous genes and is particularly useful in those situations where high level expression of a protein is disadvantageous.